1. Field of the Invention
This invention relates to a double-layered-type liquid crystal display device with the use of a twisted nematic display process that can produce a black-and-white display image with high contrast without giving rise to color and a colored display image with high contrast.
2. Description of the Prior Art
Liquid crystal display devices are now used in clocks, electronic calculating machines, computer terminals, as well as word processor displays, televisions, and a variety of other uses in many fields. Recently, there has been an extremely large demand for liquid crystal display devices because of the changes to multicolor and full-color displays, which are already being made use of in the fields of graphic display and image display. Color displays that have been widely put into practical use are attained by a liquid crystal cell with color filter layers. The liquid crystal cell functions as a light-switcher and produces various colors. The main kind of display mode is a twisted nematic (TN) display mode attained by a liquid crystal cell in which the liquid crystal molecules are twisted 90.degree.so that high contrast, etc., can be obtained. However, with this TN display mode, the dependence of the display characteristics on the wavelength of light is great, and it is not possible to achieve switching of light uniformly over the entire spectrum of visible light. In particular, with the normally closed display process in which the axes of absorption of the two polarizers are in parallel, there is the problem that light leakages at the time of the application of voltage gives rise to color.
With a color display device that causes light-switching by the use of a TN display that has this kind of a color filter layer, there are two main kinds of driving methods. One of them involves the active-matrix drive method that uses a liquid crystal cell that has picture elements provided with non-linear devices such as diodes or switching elements such as thin-film transistors. The other method involves the duty drive method in which the liquid crystals of a liquid crystal cell without picture elements are successively driven. With the latter method, the steepness in the vicinity of the threshold value of the optical characteristics of the liquid crystals is important, this is a problem with the TN displays that are now being used. In order to improve the optical characteristics so that steepness in the vicinity of the threshold value can be attained, it has been suggested that a supertwisted nematic (STN) process that gives liquid crystal molecules that are twisted at angles of around 180.degree.-270.degree. be used. With the STN process, the curve in the vicinity of the threshold value increases sharply, and even when the duty ratio increases, it is possible to obtain a high contrast ratio. However, because birefringence effects of liquid crystals are used, the dependence of the display characteristics on the wavelength is theoretically higher than with the TN display, so it is very difficult to adapt it for use in a full-color display.